Many attempts have been made previously to provide flame retardant thermoplastics, however typically plastic materials have been heavily filled with additives until the desired degree of flame retardancy has been achieved, the loadings being large enough in many instances to detract from the physical properties of the plastic base material. Several patents, such as U.S. Pat. No. 4,265,801 (Moody et al.), U.S. Pat. No. 4,235,978 (Luce et al.), U.S. Pat. No. 4,209,566 (Betts et al.) and U.S. Pat. No. 4,247,446 (Betts et al.), also describe compositions which make use of halogenated organic materials and/or heavy metal compounds (e.g., lead- or antimony-based) which produce acidic and perhaps toxic bi-products when burned.
In U.S. Pat. No. 4,273,691 (MacLaury et al.) describes flame retardant compositions comprising a polyolefin, certain metal salts of carboxylic acids and a silicone, such as a silicone gum. In commonly assigned, copending application Ser. No. 344,167, filed Jan. 29, 1982, and U.S. Pat. No. 4,387,176 (Frye), thermoplastic compositions are described which derive flame retardancy from additive packages comprising low viscosity silicone fluids plus Group IIA metal carboxylic acid salts and a blend of Group IIA metal organic compound plus silicone plus silicone resin, respectively.
All of the aforementioned patents and applications are incorporated herein by reference. It will be apparent that the present invention provides novel flame retardant compositions that represent a significant improvement over prior art compositions.
The present invention is based on the discovery that efficient flame retardant thermoplastics can be prepared by combining during compounding in the correct proportions certain metal soap precursors, for example stearic acid and a reactive magnesium compound (e.g., magnesium hydroxide or magnesium ethoxide), which are precursors to magnesium stearate, with certain silicone fluids and silicone resins and adding them to a major proportion of a thermoplastic. Such flame retarding additives impart flame retardance at relatively low concentrations, cause less reduction in mechanical properties than conventional flame retardants, dramatically improve impact resistance, are believed to produce less toxic products when exposed to flame, and act to improve gloss and processability.